The present invention relates to a curable resin composition which is suitable for an automotive top coating composition and a coil coating composition.
A top coating composition for an automobile generally comprises, as a binder component, a combination of a hydroxyl group-containing polymer and a melamine resin curing agent. However, a coated film obtained by using a melamine resin as a curing agent generally has poor acid resistance. Accordingly, such a coated film is liable to be damaged by an acid rain which has recently become a serious problem, and an appearance of the coated film is deteriorated.
The poor acid resistance which is provided by using a melamine resin is believed to originate from a triazine nucleus in the melamine resin. Accordingly, a drawback of poor acid resistance occurs, so far as a melamine resin is used as a curing agent.
In Japanese Laid-Open Patent Publication Nos. 2-45577 and 3-287650, there is suggested a novel coating composition without using a melamine resin. The coating composition comprises an acid group and an epoxy group therein, and the acid group and the epoxy group reacts to form a crosslinking point of an ester bond, and therefore, acid resistance of the resulting coated film is excellent.
The coating composition, however, has high functional group content and high viscosity. Therefore, a large amount of solvent must be used for controlling viscosity, and it is difficult to prepare a high-solid type coating composition.
On the other hand, in order to reduce the harmful influence on the environment, a high-solid coating composition which volatilizes a small amount of solvent into air has recently been required.
In Japanese Laid-Open Patent Publication No. 6-166741, there is disclosed a high-solid coating composition using a silicone polymer. In this reference, a hydroxyl group functional silicone polymer is reacted with an acid anhydride group-containing compound in order to impart acid functionality, and incorporated into the coating composition. Accordingly, an amount of functional group of the composition can not be increased, Tg of the resulting resin becomes low, and therefore, Tg of the resulting coated film can not be increased.
In Japanese Laid-Open Patent Publication No. 6-41575, there is disclosed a high-solid coating composition comprising a polyepoxide (a) and a polyester polycarboxylic acid (b). However, solid content of the coating composition is not sufficiently high in order to prevent the harmful influence on the environment, and the coated film formed has poor acid resistance.
An object of the present invention is to solve the above conventional problem, and provide a high-solid curable resin composition which forms a coated film having excellent acid rain resistance, mar resistance and appearance, as well as a high-solid coating composition and a method for forming a coated film using the same.
That is, the present invention provides a high-solid curable resin composition, comprising:
(a) 10 to 70% by weight of an acrylic polycarboxylic acid having 2 or more carboxyl groups per molecule on an average, an acid value of 5 to 300 mg KOH/g (solid) and a number-average molecular weight of 500 to 8000;
(b) 5 to 70% by weight of a polyester polycarboxylic acid having an acid value of 50 to 350 mg KOH/g (solid), a number-average molecular weight of 400 to 3500 and a ratio of weight-average molecular weight to number-average molecular weight of 1.8 or less, which is obtained by reacting a polyester polyol having 3 or more hydroxyl groups with an acid anhydride group-containing compound; and
(c) 10 to 80% by weight of a polyepoxide having an epoxy equivalent of 50 to 700 and a number-average molecular weight of 200 to 10000; provided that the amounts of the components (a) to (c) are based on the weight of the total solid contained in the curable resin composition.
An acrylic polycarboxylic acid (a) to be used in the present invention has 2 or more carboxyl groups per molecule on an average, an acid value of 5 to 300 mg KOH/g, preferably 25 to 250 mg KOH/g, more preferably 50 to 200 mg KOH/g, and a number average-molecular weight of 500 to 8000, preferably 800 to 6000, more preferably 1500 to 4000.
The acrylic polycarboxylic acid is obtained by copolymerizing 5 to 80% by weight of a carboxyl group-containing ethylenically unsaturated monomer with 20 to 95% by weight of an ethylenically unsaturated monomer having no carboxyl group according to a method known to the art. For example, the copolymerization can be carried out at a polymerization temperature of 80 to 200xc2x0 C. for a polymerization time of 3 to 10 hours under normal or applied pressure, using an azo or peroxide initiator as a radical polymerization initiator in an amount of 0.5 to 20 parts by weight, based on 100 parts by weight of the total ethylenically unsaturated monomers. Conventional polymerization controlling agents such as a chain transfer agent, a color protection agent and the like may be added during the polymerization.
Examples of the carboxyl group-containing ethylenically unsaturated monomer include acrylic acid, methacrylic acid, itaconic acid, maleic acid, an adduct of these with xcex5-caprolactone (e.g. xe2x80x9cAllonix M-5300xe2x80x9d, manufactured by Toa Gosei Kagaku Co., Ltd., etc.), an adduct of an ethylenically unsaturated monomer having a hydroxyl group represented by the formula (I) with an acid anhydride group-containing compound, and an adduct of an acid anhydride group-containing ethylenically unsaturated monomer with a monoalcohol. The ethylenically unsaturated monomer having a carboxyl group can be used alone or in combination thereof.
The acid anhydride group-containing compound is half-esterified by a hydroxyl group to provide a carboxy functional group in an ambient reaction condition such as room temperature to 150xc2x0 C. under normal pressure. It is preferred to use an acid anhydride group-containing compound having 4 to 12 carbon atoms, particularly 8 to 10 carbon atoms, which has a (unsaturated or saturated) cyclic group. Such a component may improve compatibility of the resulting resin.
Examples of the preferred acid anhydride group-containing compound include phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, 4-methyl hexahydrophthalic anhydride, trimellitic anhydride, succinic anhydride, etc.
Examples of the acid anhydride group-containing ethylenically unsaturated monomer to be used herein include itaconic anhydride, maleic anhydride, citraconic anhydride, etc.
Examples of the monoalcohol to be used herein include methanol, ethanol, n-propanol, i-propanol, n-butanol, i-butanol, t-butanol, n-hexyl alcohol, lauryl alcohol, methyl cellosolve, ethyl cellosolve, methoxypropanol, ethoxypropanol, furfuryl alcohol, dimethylamino ethanol, diethylamino ethanol, acetol, allyl alcohol, propargyl alcohol, etc.
Examples of the ethylenically unsaturated monomer having no carboxyl group include styrene, xcex1-methylstyrene, p-t-butylstyrene, (meth)acrylate (e.g. methyl(meth)acrylate, ethyl(meth)acrylate, propyl(meth)acrylate, n-, i- and t-butyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, lauryl(meth)acrylate, cyclohexyl(meth)acrylate, isobornyl(meth)acrylate), etc.), VeoVa-9 and VeoVa-10 manufactured by Shell Co., etc. When styrene or a styrene derivative is used as the ethylenically unsaturated monomer having no carboxyl group, it is preferred to use it in an amount of 5 to 40% by weight.
A preferred acrylic polycarboxylic acid (a) of the curable resin composition of the present invention, has a carboxyl group and a carboxylate group, which bond to adjacent carbon atoms each other, because acid resistance of the resulting coated film is improved. The acrylic polycarboxylic acid (a) having a carboxyl group and a carboxylate group is obtained, for example, by reacting an acrylic polyacid anhydride (a)(i) with a monoalcohol (a)(ii).
The acrylic polyacid anhydride (a)(i) is obtained by copolymerizing 15 to 40% by weight, preferably 15 to 35% by weight of an acid anhydride group-containing ethylenically unsaturated monomer (a)(i)(1) with 60 to 85% by weight, preferably 65 to 85% by weight of an ethylenically unsaturated monomer having no acid anhydride group (a)(i)(2). When an amount of the acid anhydride group-containing ethylenically unsaturated monomer (a)(i)(1) is smaller than 15% by weight, curability becomes poor. On the other hand, when the amount exceeds 40% by weight, the resulting coated film becomes too brittle, which results in poor weather resistance. Examples of the acid anhydride group-containing ethylenically unsaturated monomer (a)(i)(1) include those which have already been described.
The ethylenically unsaturated monomer having no acid anhydride group (a)(i)(2) is not specifically limited unless it exerts a harmful influence on an acid anhydride group. Preferred are those having 3 to 15 carbon atoms, particularly 3 to 12 carbon atoms, which has one ethylenically unsaturated bond.
A mixture of 2 or more ethylenically unsaturated monomers also may be used as the ethylenically unsaturated monomer having no acid anhydride group (a)(i)(2), because it is effective to improve compatibility between resins. Specific examples of the monomer (a)(i)(2) include those described above as the ethylenically unsaturated monomer having no carboxyl group.
A monomer having a carboxyl group, such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, etc. can also be used as the ethylenically unsaturated monomer having no acid anhydride group (a)(i)(2). Particularly preferred is a long-chain carboxylic acid monomer having a spacer moiety of about 5 to 20 carbon atoms between an ethylenically unsaturated group and a carboxyl group. Specific examples thereof include an adduct of the monomer having a carboxyl group with xcex5-caprolactone (e.g. Allonix M-5300). The long-chain carboxylic acid monomer may improve mar resistance of the coated film.
For example, a carboxyl group-containing ethylenically unsaturated monomer obtained by subjecting a hydroxyl group-containing ethylenically unsaturated monomer and an acid anhydride group-containing compound to the half esterification reaction in an amount that a molar ratio of a hydroxyl group to an acid anhydride group becomes 1/0.5 to 1/1.0, preferably 1/0.8 to 1/1.0, can be used as the ethylenically unsaturated monomer having no acid anhydride group (a)(i)(2). When the molar ratio is more than 1/0.5, viscosity of the polymer becomes high, which results in poor workability. On the other hand, when the molar ratio is less than 1/1.0, the excessive acid anhydride group-containing compound remains in the resulting coated film, which results in poor water resistance.
The hydroxyl group-containing ethylenically unsaturated monomer to be used herein preferably has 5 to 23 carbon atoms, more preferably 5 to 13 carbon atoms. When a carbon chain of the monomer is too short, flexibility around a crosslinking point becomes poor, and the resulting coated film becomes too hard. On the other hand, when the carbon chain is too long, a molecular weight between crosslinking points becomes too large. Generally, the hydroxyl group-containing ethylenically unsaturated monomer has a structure represented by the formula: 
wherein R is a hydrogen atom or a methyl group, and X is
an organic chain represented by the formula: 
wherein Y is a linear or branched alkylene group having 2 to 8 carbon atoms, m is an integer of 3 to 7 and q is an integer of 0 to 4, or
an organic chain represented by the formula: 
wherein R is an hydrogen atom or a methyl group and n is an integer of 2 to 50.
Examples thereof include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl(meth)acrylate, 2-hydroxybutyl(meth)acrylate, 4-hydroxybutyl(meth)acrylate, 6-hydroxyhexyl(meth)acrylate, reaction products of these with xcex5-caprolactone, and compounds prepared by esterifying (meth)acrylic acid with an excess amount of diols (e.g. 1,4-butanediol, 1,6-hexanediol, polyethylene glycol, polypropylene glycol, etc.).
These compounds are commercially available, and examples thereof include 4-hydroxybutyl acrylate xe2x80x9c4HBAxe2x80x9d and 4-hydroxybutyl methacrylate xe2x80x9c4HBMAxe2x80x9d (manufactured by Mitsubishi Chemical Co., Ltd.), and xe2x80x9cPlaccel FM1xe2x80x9d and xe2x80x9cPlaccel FA1xe2x80x9d (manufactured by Daicel Kagaku Kogyo Co., Ltd.). Examples of a propylene oxide monomer include xe2x80x9cBlemmer PP-1000xe2x80x9d and xe2x80x9cBlemmer PP-800xe2x80x9d, examples of an ethylene oxide monomer include xe2x80x9cBlemmer PE-90xe2x80x9d, manufactured by Nippon Oil and Fat Co., Ltd.
Examples of the acid anhydride group-containing compound to be used herein include phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, 4-methylhexahydrophthalic anhydride, trimellitic anhydride, succinic anhydride, etc.
The half esterification reaction between the hydroxyl group-containing ethylenically unsaturated monomer and acid anhydride group-containing compound is carried out at a temperature of room temperature to 150xc2x0 C. according to a method known to the art.
The copolymerization between the acid anhydride group-containing ethylenically unsaturated monomer (a)(i)(1) and ethylenically unsaturated monomer having no acid anhydride group (a)(i)(2) is carried out by a known method such as solution polymerization (e.g. radical polymerization, etc.). For example, it can be carried out at a polymerization temperature of 100 to 200xc2x0 C. for a polymerization time of 3 to 8 hours under normal or applied pressure. As an initiator, there can be suitably used an azo or peroxide initiator. Other additives such as chain transfer agent, etc. can also be used.
A number-average molecular weight of the resulting polymer is preferably 500 to 8000, more preferably 800 to 6000, particularly 1500 to 4000. When the number-average molecular weight exceeds 8000, compatibility between the resins becomes poor, which results in poor appearance of the coated film. On the other hand, when the number-average molecular weight is smaller than 500, curability of the resin composition becomes insufficient. The resulting polymer has at least two acid anhydride groups, preferably 2 to 15 acid anhydride groups, per molecule on an average. When the number of the acid anhydride groups per molecule is smaller than 2, curability of the resin composition becomes insufficient. On the other hand, when it exceeds 15, the resulting coated film becomes too hard and brittle, which results in poor weather resistance.
Then, the resulting acrylic polyacid anhydride (a)(i) is reacted with a monoalcohol (a)(ii) in a proportion so that a molar ratio of an acid anhydride group to a hydroxyl group becomes 1/10 to 1/1, preferably 1/5 to 1/1, more preferably 1/2.0 to 1/1 to prepare an acrylic polycarboxylic acid (a) having a carboxyl group and a carboxylate group. When the molar ratio is smaller than 1/10, an amount of the excessive alcohol is too large, which causes pinholes at the step of curing. On the other hand, the molar ratio exceeds 1/1, the excessive anhydride group remains in the resulting resin composition, and storage stability becomes poor.
It is preferred that the monoalcohol (a)(ii) which can be used in the present invention has 1 to 12 carbon atoms, particularly 1 to 8 carbon atoms. Such a monoalcohol evaporates when heating, and is convenient for regenerating an acid anhydride group. Examples of the preferred monoalcohol include those which have already been described. Particularly preferred examples include acetol, furfuryl alcohol, allyl alcohol, propargyl alcohol, ethanol and methanol.
The resulting acrylic polycarboxylic acid (a) having a carboxyl group and a carboxylate group has an acid value of 5 to 300 mg KOH/g, preferably 50 to 250 mg KOH/g. When the acid value is smaller than 5 mg KOH/g, curability of the resulting resin composition becomes insufficient. On the other hand, when it exceeds 300 mg KOH/g, storage stability becomes poor.
An acrylic polycarboxylic acid (a) component can be contained in an amount of 10 to 70% by weight, preferably 15 to 50% by weight, more preferably 20 to 45% by weight, based on weight of the total solid contained in the curable resin composition. When an amount of the acrylic carboxylic acid component (a) is smaller than 10% by weight, acid resistance of the resulting coated film becomes poor. On the other hand, when it exceeds 70% by weight, the coated film becomes too hard.
A polyester polycarboxylic acid (b) to be used for the curable resin composition of the present invention is obtained by subjecting a polyester polyol having 3 or more hydroxyl groups and an acid anhydride group-containing compound to the half esterification reaction.
In the present specification, the term xe2x80x9cpolyester polyolxe2x80x9d means a polyhydric alcohol having 2 or more ester moieties. The term xe2x80x9cpolyhydric alcoholxe2x80x9d means an alcohol having 2 or more hydroxyl groups.
The polyester polyol to be used herein reacts with an acid anhydride group-containing compound, and provides a polyester polycarboxylic acid having 2 or more acid functional groups per molecule. The polyester polycarboxylic acid has the following characteristics.
The polyester polyol is generally prepared by condensing a low-molecular weight polyhydric alcohol having 3 to 16 carbon atoms and having 3 or more hydroxyl groups, with a linear aliphatic dicarboxylic acid. As a result, a linear aliphatic group is introduced into the low-molecular weight polyhydric alcohol, thereby flexibility of the resulting coated films is improved, and impact resistance is improved.
Examples of the low-molecular weight polyhydric alcohol which can be used include trimethylolpropane, trimethylolethane, 1,2,4-butanetriol, ditrimethylolpropane, pentaerythritol, dipentaerythritol, glycerol and mixtures thereof.
Examples of the dicarboxylic acid include a dibasic acid such as phthalic acid, isophthalic acid, terephthalic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, tetrahydrophthalic acid, hexahydrophthalic acid, maleic acid and fumaric acid, and mixtures thereof. There can also be used an acid anhydride group-containing compound such as succinic anhydride, phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, hymic anhydride, trimellitic anhydride, methylcyclohexenetricarboxylic anhydride and pyromellitic anhydride, and mixtures thereof.
The polyester polyol is synthesized by a conventional esterification reaction. That is, polyesterification proceeds on a condensation reaction between a polyhydric alcohol and a polybasic acid, or esterification between a polyhydric alcohol and an acid anhydride group-containing compound and further dehydration with an alkyl component. A polyester oligomer having comparatively low molecular weight is obtained by the procedure, and a high-solid resin composition may be provided.
A polyester polyol which is particularly preferred to be used in the present invention is obtained by adding a lactone compound such as xcex5-caprolactone to the low-molecular weight polyhydric alcohol, and extending a chain of the alcohol. The resulting polyester polyol has narrow molecular weight distribution, and it may provide a high-solid resin composition and a coated film having good weather resistance and water resistance. Examples of the low-molecular weight polyhydric alcohol which is preferably used, include trimethylolpropane, di-trimethylolpropane, pentaerythritol, etc.
The xe2x80x9clactone compoundxe2x80x9d to be used in the present invention is a cyclic compound having an oxygen atom in the ring, and it reacts with a nucleophilic reagent to open its ring and to form a terminal hydroxyl group. Preferred lactone compound has 4 to 7 carbon atoms, because it easily react to open its ring.
Examples thereof include xcex5-caprolactone, xcex3-caprolactone, xcex3-valerolactone, xcex4-valerolactone, xcex3-butyrolactone, etc. Among them, xcex5-caprolactone, xcex3-valerolactone and xcex3-butyrolactone are preferably used.
The chain extension reaction can be carried out under the same condition as that of a conventional ring opening addition reaction. For example, a chain extended polyester polyol is derived from the low-molecular weight polyhydric alcohol, by reacting at a temperature of 80 to 200xc2x0 C. in the presence or absence of a suitable solvent for 5 hours or less. A tin compound catalyst and the like may also be used.
A molar amount of the lactone compound is 0.2- to 10-fold, preferably 0.25- to 5-fold, more preferably 0.3- to 3-fold, based on a molar amount of OH group of the low-molecular weight polyhydric alcohol. When a molar amount of the lactone compound based on that of OH group is smaller than 0.2-fold, the resin becomes hard, which results in poor impact resistance of the coated film. On the other hand, when it exceeds 10-fold, hardness of the coated film becomes poor.
A polyester polycarboxylic acid (b) to be used for the curable resin composition of the present invention has an acid value of 50 to 350, preferably 100 to 300, more preferably 150 to 250 mg KOH/g (solid content), a number average-molecular weight of 400 to 3500, preferably 500 to 2500, more preferably 700 to 2000 and a ratio of weight-average molecular weight to number-average molecular weight of 1.8 or less, preferably 1.5 or less, more preferably 1.35 or less.
When the acid value exceeds 350, viscosity of the resin composition becomes too high, thereby solid content of the resin composition is lowered. On the other hand, when the acid value is smaller than 50, curability of the resin composition becomes poor. When the molecular weight exceeds 3500, viscosity of the resin composition becomes high and the handling becomes difficult, thereby solid content of the resin composition is lowered. On the other hand, when the molecular weight is smaller than 400, curability of the resin composition becomes poor or the water resistance of the coated film becomes poor. When the weight average-molecular weight/number average-molecular exceeds 1.8, water resistance or weather resistance of the coated film becomes poor.
The half esterification reaction between the polyester polyol and the acid anhydride group-containing compound can be carried out by using the acid anhydride group-containing compound such as phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, 4-methylhexahydrophthalic anhydride, trimellitic anhydride, succinic anhydride, etc. under an ambient reaction condition such as room temperature to 150xc2x0 C. and normal pressure. It is not necessary to convert all hydroxyl groups of the polyester polyol into carboxyl groups, but a part of the hydroxyl groups may remain.
A polyester polycarboxylic acid having a hydroxyl group provides both a carboxyl group and a hydroxyl group on a surface of the coated film. Therefore, in case of recoating, it provides excellent adhesiveness, in comparison with a polyester polycarboxylic acid having no hydroxyl group.
The polyester polycarboxylic acid (b) may have a hydroxyl value of not more than 150 mg KOH/g (solid content), preferably 5 to 100 mg KOH/g, more preferably 10 to 80 mg KOH/g. When the hydroxyl value exceeds 150 mg KOH/g, water resistance of the coated film becomes poor.
The polyester polycarboxylic acid having a hydroxyl group and a carboxyl group can react and bond with both polyepoxide (c) and acrylic polycarboxylic acid (a), and therefore, a strong coated film can be obtained. Those having 0.1 or more hydroxyl groups per molecule on an average are preferred.
It is desired that a molar amount of an acid anhydride group of the acid anhydride group-containing compound is 0.2- to 1.0-fold, particularly 0.5- to 0.9-fold based on a molar amount of OH group of the polyester polyol. When a molar amount of the acid anhydride group based on the molar amount of OH group is smaller than 0.2-fold, curability of the resulting resin composition becomes poor.
A polyester polycarboxylic acid (b) component can be contained in an amount of 5 to 70% by weight, preferably 5 to 50% by weight, more preferably 10 to 40% by weight, based on weight of the total solid contained in the curable resin composition. When an amount of the polyester polycarboxylic acid is smaller than 5% by weight, solid content of the resin composition is not increased. On the other hand, when it exceeds 70% by weight, weather resistance of the coated film becomes poor.
A polyepoxide (c) to be used for the curable resin composition of the present invention has 2 or more epoxy groups, preferably 2 to 10 epoxy groups, more preferably 3 to 8 epoxy groups per molecule on an average.
Examples thereof include a glycidyl ether of a polyhydric alcohol and a glycidyl ester of a polybasic acid, such as glycerol triglycidyl ether, trimethylolpropane triglycidyl ether, pentaerythritol tetraglycidyl ether, sorbitol hexaglycidyl ether, diglycidyl hexahydrophthalate, etc.
A number-average molecular weight of the polyepoxide is 200 to 10000, preferably 500 to 8000, more preferably 800 to 5000. When a number-average molecular weight of the acrylic polyepoxide is smaller than 200, curability of the resulting coated film becomes poor. On the other hand, when it exceeds 10000, solid content of the resulting resin composition becomes low. In addition, an epoxy equivalent is 50 to 700, preferably 80 to 600, more preferably 100 to 500. When an epoxy equivalent of the acrylic polyepoxide is larger than the above upper limit, curability of the resin composition becomes poor. On the other hand, when the epoxy equivalent is smaller than the lower limit, the coated film becomes hard and brittle, and it is not preferred.
A polyepoxide which is preferably used in the present invention is an acrylic polyepoxide obtained by copolymerizing 10 to 60% by weight, preferably 15 to 50% by weight of an epoxy group-containing ethylenically unsaturated monomer with 40 to 90% by weight, preferably 10 to 60% by weight of an ethylenically unsaturated monomer having no epoxy group. When an amount of the epoxy group-containing ethylenically unsaturated monomer is smaller than 10% by weight, curability becomes poor. On the other hand, when the amount is larger than 60% by weight, the coated film becomes too hard and weather resistance becomes poor.
Examples of the epoxy group-containing ethylenically unsaturated monomer include glycidyl (meth)acrylate, xcex2-methylglycidyl(meth)acrylate, 3,4-epoxycyclohexanyl(meth)acrylate, etc. It is preferred to use glycidyl(meth)acrylate so as to obtain a resin composition having good balanced curability and storage stability.
Examples of the ethylenically unsaturated monomer having no epoxy group include those described above as the ethylenically unsaturated monomer having no acid anhydride group (a)(i)(2) which is used for preparing the acrylic polyacid anhydride (a)(i). The copolymerization can also be carried out according to the same manner as that described above.
A number-average molecular weight of the acrylic polyepoxide is 500 to 10000, preferably 1000 to 8000, more preferably 1500 to 5000. When a number-average molecular weight of the acrylic polyepoxide is smaller than 500, curability of the resulting coated film becomes poor. On the other hand, when it exceeds 10000, solid content of the resulting resin composition becomes low. In addition, an epoxy equivalent is 50 to 700, preferably 80 to 600, more preferably 100 to 500. When an epoxy equivalent of the acrylic polyepoxide is larger than the above upper limit, curability of the resin composition becomes poor. On the other hand, when the epoxy equivalent is smaller than the lower limit, the coated film becomes hard and brittle, and it is not preferred.
The above-described hydroxyl group-containing ethylenically unsaturated monomer which is used for preparing the ethylenically unsaturated monomer having no acid anhydride group (a)(i)(2) can also be used as the ethylenically unsaturated monomer having no epoxy group.
When using the ethylenically unsaturated monomer having a hydroxyl group as the ethylenically unsaturated monomer having no epoxy group, adhesiveness and recoating properties of the resulting coated film are improved. In addition, the acrylic polyepoxide having a hydroxyl group and a carboxylate group, which is obtained by using the ethylenically unsaturated monomer having a hydroxyl group as the ethylenically unsaturated monomer having no epoxy group, reacts and bonds with an acrylic polycarboxylic acid (a) having a carboxyl group and a carboxylate group, in both functional groups (e.g. hydroxyl group and epoxy group), as described hereinafter. Therefore, a strong coated film can be obtained.
A hydroxyl value of the resulting acrylic polyepoxide is 5 to 300 mg KOH/g, preferably 10 to 200 mg KOH/g, more preferably 15 to 150 mg KOH/g. When the hydroxyl value exceeds 300, solid content of the resin composition is lowered and water resistance of the cured coated film becomes poor. On the other hand, when it is smaller than 5, adhesiveness of the coated film becomes poor.
Particularly preferred polyepoxide (c) to be used in the present invention is obtained by copolymerizing
(i) 5 to 70% by weight of a hydroxyl group-containing ethylenically unsaturated monomer having a structure represented by the formula: 
wherein R is a hydrogen atom or a methyl group, and X is
an organic chain represented by the formula: 
wherein Y is a linear or branched alkylene group having 2 to 8 carbon atoms, m is an integer of 3 to 7 and q is an integer of 0 to 4, or
an organic chain represented by the formula: 
wherein R is an hydrogen atom or a methyl group and n is an integer of 2 to 50;
(ii) 10 to 60% by weight of an epoxy group-containing ethylenically unsaturated monomer; and
(iii) 0 to 85% by weight of an optional ethylenically unsaturated monomer having neither hydroxyl nor epoxy group.
The polyepoxide (c) having a hydroxyl group and an epoxy group has preferably 2 to 12 epoxy groups, more preferably 3 to 10 epoxy groups and 0.5 to 10 hydroxyl groups, more preferably 1 to 8 hydroxyl groups per molecule on an average.
A polyepoxide (c) component can be contained in an amount of 10 to 80% by weight, preferably 20 to 70% by weight, more preferably 30 to 65% by weight, based on weight of the total solid contained in the curable resin composition. When an amount of the polyepoxide is smaller than 10% by weight, curability of the resulting coated film becomes poor. On the other hand, when it exceeds 70% by weight, yellowing resistance of the coated film becomes poor.
Thus obtained acrylic polycarboxylic acid (a), polyester polycarboxylic acid (b) and polyepoxide (c) were formulated to obtain a curable resin composition of the present invention.
The formulation of the acrylic polycarboxylic acid (a), polyester polycarboxylic acid (b) and polyepoxide (c) can be carried out according to an amount and a method known to those skilled in the art. When using an acrylic polycarboxylic acid having a carboxyl group and a carboxylate group as the acrylic polycarboxylic acid (a) and using a polyepoxide having a hydroxyl group and an epoxy group as the polyepoxide (c), there can be obtained a high-solid curable resin composition which forms a coated film having good acid resistance.
In that case, it is preferred to formulate in an amount so that a molar ratio of a carboxyl group contained in the acrylic polycarboxylic acid (a) and the polyester polycarboxylic acid (b) to an epoxy group contained in the polyepoxide (c) becomes 1/1.4 to 1/0.6, preferably 1/1.2 to 1/0.8, and a molar ratio of a carboxyl group or a carboxylate group which bonds to a carbon atom adjacent to a carbon atom bonding the carboxyl group contained in the acrylic polycarboxylic acid (a) to a hydroxyl group contained in the polyester polycarboxylic acid (b) and the polyepoxide (c) becomes 1/2.0 to 1/0.5, more preferably 1/1.5 to 1/0.7.
When a ratio of a carboxyl group contained in the acrylic polycarboxylic acid (a) and the polyester polycarboxylic acid (b) to an epoxy group contained in the polyepoxide (c) exceeds 1/0.6, curability of the resulting resin composition becomes poor. On the other hand, when it is smaller than 1/1.4, the coated film may become yellow. When a ratio of a carboxyl or a carboxylate group contained in the acrylic polycarboxylic acid (a) to a hydroxyl group contained in the polyester polycarboxylic acid (b) and the polyepoxide (c) exceeds 1/0.5, curability of the resulting resin composition becomes poor. On the other hand, when it is smaller than 1/2.0, water resistance of the coated film becomes poor because of excess hydroxyl groups. An amount of each component for the formulation can be calculated from hydroxyl value, acid value and epoxy equivalent of the polymers according to the calculation method known to those skilled in the art.
Curing mechanism of a curable resin composition of the present invention thus obtained is as follows. That is, a carboxyl group and a carboxylate group in the acrylic polycarboxylic acid (a) react together by heating, and form an acid anhydride group and a free monoalcohol in the acrylic polycarboxylic acid (a). The monoalcohol evaporates and leaves from the system. The acid anhydride group formed in the acrylic polycarboxylic acid (a) reacts with a hydroxyl group contained in the polyester polycarboxylic acid (b) and the polyepoxide (c), and forms a crosslinking point and a carboxyl group again. This carboxyl group and a carboxyl group which is present in the polyester polycarboxylic acid (b) react with an epoxy group in the polyepoxide (c), and form a crosslinking point. In such way, three sorts of polymers reacts together to cure and high density of crosslinking is provided.
In addition to the above essential components, a binder component such as silicone polymer can be optionally formulated in the curable resin composition of the present invention, because it is effective for increasing solid content of the resulting resin composition.
A silicone polymer which can be used in the present invention is a silicone polymer having an epoxy group and/or an alkoxy group, which is represented by the formula: 
wherein R1 to R6 independently represents a substituent selected from the group consisting of an alkyl group having 1 to 10 carbon atoms, a phenyl group having 1 to 10 carbon atoms, a phenethyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 5 carbon atoms and the group. represented by the formulas:
R7xe2x80x94Si(OR8)3, 
R7xe2x80x94Si(OR8)2CH3, 
R7xe2x80x94Si(OR8)(CH3)2 and 
R7xe2x80x94Y 
wherein R7 represents a linear or branched alkylene group or a linear or branched alkylene group having an ether or ester moiety, R8 represents an alkyl group having 1 to 5 carbon atoms, and Y represents an aliphatic or cycloalkyl group having an epoxy group,
provided that at least one of R1 to R6 is an alkoxy group and another at least one is epoxy group; 1 represents an integer of 1 to 20; m represents an integer of 0 to 4; and n represents an integer of 0 to 2. A sequence of each repeating unit constituting the silicone polymer may be arranged at random, and is not limited to the order of the formula (II).
Examples of a silicone polymer having an epoxy group include xe2x80x9cNUC Siliconexe2x80x9d series manufactured by Nippon Unicar Co., Ltd. Examples of a silicone polymer having an alkoxy group include xe2x80x9cKC89-Sxe2x80x9d manufactured by Shin-Etsu Chemical Kogyo Co., Ltd. Examples of a silicone polymer having an epoxy group and an alkoxy group include xe2x80x9cMKC Silicate MSEP 2xe2x80x9d series manufactured by Mitsubishi Chemical Co., Ltd. and xe2x80x9cNUC Siliconexe2x80x9d series manufactured by Nippon Unicar Co., Ltd.
A method for producing the silicone polymers is described in xe2x80x9cThe 1990 Organic Silicone Material Chemical Symposium, preliminary manuscript, pages 29 to 30xe2x80x9d. An epoxy group may be pendent from an aliphatic or alicyclic hydrocarbon chain or it may be present at terminal end thereof. In the silicone polymer (II), the aliphatic or alicyclic hydrocarbon chain having an epoxy group Y is represented, for example, by the following formula: 
wherein R11, R12 and R13 represent a hydrocarbon having up to 4 carbon atoms.
In the present specification, the term xe2x80x9cepoxy equivalentxe2x80x9d means a weight in grams of a compound having 1 gram equivalent of an epoxy group. The term xe2x80x9calkoxy equivalentxe2x80x9d means a weight in grams of a compound having 1 gram equivalent of an alkoxy group. The term xe2x80x9chydroxyl equivalentxe2x80x9d means a weight in grams of a compound having 1 gram equivalent of a hydroxyl group.
An epoxy equivalent of this silicone polymer is 100 to 1500, and an alkoxy equivalent is 50 to 1500. When the epoxy equivalent is less than 100, storage stability of the resin composition becomes poor. On the other hand, when it exceeds 1500, curability becomes poor. A preferable range of the epoxy equivalent is 140 to 1000, more preferably 180 to 700. A preferable range of the alkoxy equivalent is 60 to 800, more preferably 80 to 500.
The silicone polymer component having an epoxy group and an alkoxy group can be formulated in an amount of not more than 30% by weight, preferably 1 to 20% by weight, more preferably 3 to 15% by weight. When an amount of the silicone polymer component exceeds 30% by weight, storage stability of the resulting resin composition becomes poor.
In the present invention, a silicone polymer having a hydroxyl group and a carboxyl group can be used together with or in place of the silicone polymer component having an epoxy group and/or an alkoxy group. The silicone polymer having a hydroxyl group and a carboxyl group is obtained by subjecting a silicone polymer having a hydroxyl group and an acid anhydride group-containing compound to the half esterification reaction.
A number-average molecular weight of this silicone polymer is 500 to 6000, preferably 1000 to 4500. A hydroxyl value is 2 to 120, preferably 10 to 80. An acid value is 20 to 180, preferably 35 to 150. When the number-average molecular weight, hydroxyl value or acid value exceeds the upper limit of the above range, it becomes difficult to prepare a resin composition having satisfactory high solid content. On the other hand, when it is smaller than the lower limit, curability of the resulting resin composition becomes poor.
A silicone polymer having a hydroxyl group is commercially available, and examples thereof include KR-2001 manufactured by Shin-Etsu Chemical Co., Ltd, NUC-Silicone series manufactured by Nippon Unicar Co., Ltd., represented by the following formula: 
It is preferred that a silicone polymer having a hydroxyl group has 3 to 12 hydroxyl groups per molecule on an average. When the number of hydroxyl groups is less than 3, curability becomes poor. On the other hand, when it exceeds 12, viscosity becomes high and it becomes difficult to increase a solid content of the resulting resin composition.
The acid anhydride group-containing compound is half-esterified by a hydroxyl group to provide a carboxy functional group, in an ambient reaction condition such as room temperature to 150xc2x0 C. under normal pressure. It is preferred to use an acid anhydride group-containing compound having 8 to 12 carbon atoms, which has a (unsaturated or saturated) cyclic group. Such a component may improve compatibility of the resulting resin.
Examples of the preferred acid anhydride group-containing compound include hexahydrophthalic anhydride, phthalic anhydride, tetrahydrophthalic anhydride, 4-methyl hexahydrophthalic anhydride, trimellitic anhydride, etc.
The half esterification reaction between the hydroxyl group-containing silicone polymer and the acid anhydride group-containing compound is carried out at a temperature of room temperature to 120xc2x0 C. for 30 minutes to 8 hours according to a conventional procedure. When the reaction is carried out for a long period of time at a temperature of more than 120xc2x0 C., a polyesterification reaction occurs and a high-molecular weight silicone polyester may be formed. Since such a high-molecular weight silicone polyester has small amount of functional groups and has high viscosity, it is not preferred to use in the present invention.
A silicone polymer component having a hydroxyl group and a carboxyl group can be formulated in an amount of not more than 30% by weight, preferably 1 to 20% by weight, more preferably 3 to 15% by weight. When an amount of the silicone polymer component exceeds 30% by weight, storage stability of the resulting resin composition becomes poor.
When a silicone polymer component having an epoxy group and an alkoxy group and a silicone polymer component having a hydroxyl group and a carboxyl group is used in combination, they can be formulated in an amount that the sum of them is not more than 30% by weight, preferably 1 to 20% by weight, more preferably 3 to 15% by weight based on weight of the total solid contained in the curable resin composition. When an amount of the silicone polymer component exceeds 30% by weight, storage stability of the resulting resin composition becomes poor.
In addition to the above components, a curable resin composition of the present invention may contain a curing catalyst which is generally used for the esterification reaction between an acid and an epoxy, such as quaternary ammonium salts. Examples of the other catalyst which can be used for a curable resin composition of the present invention include benzyltriethylammonium chloride or bromide, tetrabutylammonium chloride, bromide, salicylate or glycolate, paratoluenesulfonate, etc. These curing catalysts may be used in combination thereof.
A curing catalyst is generally used in an amount of 0.01 to 3.0 by weight, preferably 0.1 to 1.5% by weight, more preferably 0.4 to 1.2% by weight. When an amount of the curing catalyst is smaller than 0.01% by weight, curability becomes poor. On the other hand, when it exceeds 3.0% by weight, storage stability becomes poor.
In addition, a tin compound may be used in combination with these catalysts, as is described in Japanese Laid-Open Patent Publication Nos. 2-151651 and 2-279713. Examples of the tin catalyst include dimethyltin bis-(methyl maleate), dimethyltin bis-(ethyl maleate), dimethyltin bis-(butyl maleate), dibutyltin bis-(butyl maleate), etc.
The tin compound can be generally used in an amount of 0.05 to 6.0% by weight, preferably 0.1 to 4.0% by weight, more preferably 0.2 to 2.0% by weight. When an amount of the tin compound is smaller than 0.05% by weight, storage stability becomes poor. On the other hand, when it exceeds 6.0% by weight, weather resistance becomes poor. When the curing catalyst and the tin compound are used in combination, it is preferred that the weight ratio of the curing catalyst to the tin compound is 1/4 to 1/0.2.
In order to increase crosslinking density and to improve water resistance, a blocked isocyanate may be added to a resin composition of the present invention. In order to improve weather resistance of the coated film, an ultraviolet absorber, a hindered amine photostabilizer and an antioxidant may be added. Crosslinked resin particles for controlling rheology, and a surface modifier for modifying appearance of the coated film may also be added. Furthermore, in order to control viscosity, alcoholic solvents (e.g. methanol, ethanol, propanol, butanol, etc.) and hydrocarbon and ester solvents may be used.
When the crosslinked resin particles are used, they are added in an amount of 0.01 to 10 parts by weight, preferably 0.1 to 5 parts by weight, based on 100 parts by weight of the resin solid of the curable resin composition of the present invention. When an amount of the crosslinked resin particles exceeds 10 parts by weight, appearance of the coated film becomes poor. On the other hand, when it is smaller than 0.01 parts by weight, no rheology controlling effect is obtained.
A resin to be used in the present invention has an acid group as a functional group. Accordingly, it is also possible to prepare an aqueous resin composition comprising water as a medium by neutralizing the acid group with amine.
The present invention also provides a coating composition comprising above-described curable resin composition as a binder component. A method for preparing the coating composition of the present invention is not specifically limited, and there can be used all methods which are known to those skilled in the art.
A coating composition of the present invention can be coated by spray coating, brush coating, dip coating, roll coating, curtain coating, etc. A substrate may be optionally undercoated or intercoated. A known coating composition can be used for undercoating or intercoating the substrate.
A coating composition of the present invention can be advantageously used for any substrate such as wood, metal, glass, fabric, plastic, foam, etc., particularly plastic and surface of metal such as steel, aluminum and alloys thereof. Generally, thickness of the coated film varies depending on the desired application. A film thickness of 0.5 to 3 mills is useful in almost all cases.
After applying the coating composition on the substrate, the resulting coating is cured. High crosslinking density is formed by curing at 100 to 180xc2x0 C., preferably 120 to 160xc2x0 C. A curing time varies depending on a curing temperature used, but is usually for 10 to 30 minutes at 120 to 160xc2x0 C.
In one preferred embodiment of the present invention, a coated film is provided according to the process which comprises the steps of:
applying a water-based or solvent-based coating composition on an undercoated or intercoated substrate to form a base coating;
applying a clear coating composition comprising an acrylic polycarboxylic acid, a polyester polycarboxylic acid and a polyepoxide as a binder thereon to form a clear coating, without curing the base coating; and
heating and curing both the base coating and clear coating.
As described above, according to the present invention, there is provided a high-solid content coating composition which is superior in acid rain resistance, mar resistance and appearance.